As is well known, drilling fluid is utilized in well-drilling operations for a number of basic purposes. One purpose is to cool and lubricate the bit and the string. Another is to carry up to the surface the bore hole material which is produced as a result of the drilling operation. A third purpose is to deposit a tough and low-permeability filter cake against the sides of the bore-hole and thus reduce the invasion of the fluid phase into the formation and control fluid losses down hole. A fourth is to overbalance formation pore-pressures with sufficient hydrostatic head in order to control well flowing. A fifth is for control of corrosion of the drill string and bit. A sixth is to buoyantly support the drill string.
Due to geothermal heat in the surrounding formations, the temperature of the drilling fluid can rise as high as 600.degree. F. or more. The pressure of the drilling fluid is a function of depth and density. For very deep wells, the pressure of the drilling fluid at the bottom of the well can be as high as 20,000 psi or more.
It is very important for the drilling fluid not only to deposit a low permeability filter cake on the borehole wall, but one which is sufficiently tough and cohesive to resist erosion by the upwardly flowing drilling fluid in contact with the borehole walls.
The co-pending U.S. application Ser. No. 551,343, filed Nov. 14, 1983, is directed to an apparatus which comprises a main housing, a permeable cylindrical sleeve fixed with respect to the main housing and simulating a rock formation through which a well is drilled, first means defining a compartment exterior to the sleeve, second means for introducing into the interior of the sleeve a solids-containing fluid and for raising the fluid to a pressure higher than the pressure in the compartment, whereby the fluid passes through the sleeve and into the compartment, leaving a filter cake on the inside of the sleeve, and a probe extending through the sleeve and simulating a drill string. The apparatus is capable of depositing a filter cake and of making various measurements under controlled hydraulic regimes.
The invention set forth in the parent U.S. application Ser. No. 594,116 (of which this is a continuation-in-part) pertains to an apparatus for cooperation with the apparatus just described, which allows a determination of the soil mechanics properties of the filter cake, and the profile of filter cake thickness measured circumferentially around the sleeve. The apparatus is also capable of off-centering the probe, which may induce a non-uniform filter cake thickness.
One of the capabilities of the apparatus described in both of the aforesaid applications is that of determining the resistance of the filter cake to erosion by the flowing stream of drilling fluid. The drilling fluid is made to flow through the permeable cylindrical sleeve by a recirculating pump, typically a positive displacement pump, and this forced flow carries the drilling fluid along the surface of the forming filter cake.
If the volumetric flow setting of the pump were to remain constant while the filter cake gradually built up against the permeable cylindrical sleeve, the cross-section at the sleeve available for flow would constantly decrease, thus causing higher and higher velocities of the drilling fluid past the filter cake. In actual drilling conditions the flow speed and the shear rate at the outside surface of the filter cake tend to be constant, as long as rheological properties, hole size, pipe size and flow rate remain constant. Thus, in order to improve the simulation of down-hole conditions, the shear rate at the surface of the filter cake in the simulator should remain constant, regardless of filter cake build-up.